1 javacup javacup (construct useful parser) is a parser generator produce a parser written in java,...

1

JavaCUP

• JavaCUP (Construct Useful Parser) is a parser generator• Produce a parser written in java, itself is also written in

Java;• There are many parser generators.

– YACC (Yet Another Compiler-Compiler) for C programming language (dragon book chapter 4.9);

• There are also many parser generators written in Java– JavaCC;– ANTLR;

2

More on classification of java parser generators

• Bottom up Parser Generators Tools – JavaCUP;– jay, YACC for Java www.inf.uos.de/bernd/jay– SableCC, The Sable Compiler Compiler www.sablecc.org

• Topdown Parser Generators Tools– ANTLR, Another Tool for Language Recognition www.antlr.org– JavaCC, Java Compiler Compiler www.webgain.com/java_cc

3

What is a parser generator

Total := price + tax ;

Scanner

Parser

id

Exp + id

Expr

assignment

:= id

T o t a l : = p r i c e + t a x ;

Parser generator (JavaCup)

Context Free Grammar

4

Steps to use JavaCup

• Write a javaCup specification (cup file)– Defines the grammar and actions in a file (say, calc.cup)

• Run javaCup to generate a parser– java java_cup.Main calc.cup– Notice the package prefix java_cup before Main;– Will generate parser.java and sym.java (default class names,

which can be changed)

• Write your program that uses the parser– For example, UseParser.java

• Compile and run your program

5

Example 1: parse an expression and evaluate it

• Grammar for arithmetic expression– exprexpr ‘+’ expr | expr ‘–’ expr | expr ‘*’ expr | expr ‘/’expr |

‘(‘expr’)’ | number

• Example– (2+4)*3

• Our tasks:– Tell whether an expression like “(2+4)*3” is syntactically correct;– Evaluate the expression. (we are actually producing an interpreter

for the “expression language”).

6

The overall picture

JLex

CalcScanner

javaCup

CalcParser

calc.lex calc.cup

expression

(2+4)*3

tokens

SymbolScanner

CalcScanner CalcParser

lr_parser

implements extends

java_cup.runtime

result

CalcParserUser

public interface Scanner { public Symbol next_token() throws java.lang.Exception;}

7

Calculator javaCup specification (calc.cup)terminal PLUS, MINUS, TIMES, DIVIDE, LPAREN, RPAREN;terminal Integer NUMBER;non terminal Integer expr;precedence left PLUS, MINUS;precedence left TIMES, DIVIDE;expr ::= expr PLUS expr | expr MINUS expr | expr TIMES expr | expr DIVIDE expr | LPAREN expr RPAREN | NUMBER ;

• Is the grammar ambiguous? • Add precedence and associativity

– left means, that a + b + c is parsed as (a + b) + c– lowest precedence comes first, so a + b * c is parsed as a + (b * c)

• How can we get PLUS, NUMBER, ...? – They are the terminals returned by the scanner.

• How to connect with the scanner?

8

Ambiguous grammar error

• If we enter the grammar as below:Expression ::= Expression PLUS Expression;

• Without precedence JavaCUP will tell us:Shift/Reduce conflict found in state #4

between Expression ::= Expression PLUS Expression ()

and Expression ::= Expression () PLUS Expression

under symbol PLUS

Resolved in favor of shifting.

• The grammar is ambiguous!• Telling JavaCUP that PLUS is left associative helps.

9

Corresponding scanner specification (calc.lex)1.import java_cup.runtime.Symbol;2.Import java_cup.runtime.Scanner;3.%%4.%implements java_cup.runtime.Scanner5.%type Symbol6.%function next_token7.%class CalcScanner8.%eofval{ return null;9.%eofval}10.NUMBER = [0-9]+11.%%12."+" { return new Symbol(CalcSymbol.PLUS); }13."-" { return new Symbol(CalcSymbol.MINUS); }14."*" { return new Symbol(CalcSymbol.TIMES); }15."/" { return new Symbol(CalcSymbol.DIVIDE); }16.{NUMBER} { return new Symbol(CalcSymbol.NUMBER, new Integer(yytext()));} 17.\r|\n|. {}

• Connection with the parser– imports java_cup.runtime.*, Symbol, Scanner. – implements Scanner– next_token: defined in Scanner interface– CalcSymbol, PLUS, MINUS, ...– new Integer(yytext())

10

Run JLex

D:\214>java JLex.Main calc.lex– note the package prefix JLex – program text generated: calc.lex.java

D:\214>javac calc.lex.java– classes generated: CalcScanner.class

11

Generated CalcScanner class1. import java_cup.runtime.Symbol; 2. Import java_cup.runtime.Scanner;3. class CalcScanner implements java_cup.runtime.Scanner {4. ... .... 5. public Symbol next_token () {6. ... ... 7. case 3: { return new Symbol(CalcSymbol.MINUS); }8. case 6: { return new Symbol(CalcSymbol.NUMBER, new

Integer(yytext()));}9. ... ...10. }11. }

• Interface Scanner is defined in java_cup.runtime packagepublic interface Scanner { public Symbol next_token() throws

java.lang.Exception;}

12

Run javaCup• Run javaCup to generate the parser

– D:\214>java java_cup.Main -parser CalcParser -symbols CalcSymbol calc.cup

– classes generated: • CalcParser;

• CalcSymbol;

• Compile the parser and relevant classes– D:\214>javac CalcParser.java CalcSymbol.java

CalcParserUser.java

• Use the parser– D:\214>java CalcParserUser

13

The token class Symbol.java

1. public class Symbol {2. public int sym, left, right; 3. public Object value;4. public Symbol(int id, int l, int r, Object o) { 5. this(id); left = l; right = r; value = o;6. }7. ... ...8. public Symbol(int id, Object o) { this(id, -1, -1, o); }9. public String toString() { return "#"+sym; }10. }

• Instance variables: – sym: the symbol type;– left: left position in the original input file;– right: right position in the original input file;– value: the lexical value.

• Recall the action in lex file:return new Symbol(CalcSymbol.NUMBER, new Integer (yytext()));

14

CalcSymbol.java (default name is sym.java)1. public class CalcSymbol {2. public static final int MINUS = 3;3. public static final int DIVIDE = 5;4. public static final int NUMBER = 8;5. public static final int EOF = 0;6. public static final int PLUS = 2;7. public static final int error = 1;8. public static final int RPAREN = 7;9. public static final int TIMES = 4;10. public static final int LPAREN = 6;11.}• Contain token declaration, one for each token (terminal); Generated from the

terminal list in cup file–terminal PLUS, MINUS, TIMES, DIVIDE, LPAREN, RPAREN;–terminal Integer NUMBER

• Used by scanner to refer to symbol types, e.g., –return new Symbol(CalcSymbol.PLUS);

• Class name comes from –symbols directive. java java_cup.Main -parser CalcParser -symbols CalcSymbol calc.cup

15

The program that uses the CalcPaser

1. import java.io.*;

2. class CalcParserUser {

3. public static void main(String[] args) throws IOException{

4. File inputFile = new File ("d:/214/calc.input");

5. CalcParser parser= new CalcParser

6. (new CalcScanner(new FileInputStream(inputFile)));

7. parser.parse();

8. }

9. }

• The input text to be parsed can be any input stream (in this example it is a FileInputStream);

• The first step is to construct a parser object. A parser can be constructed using a scanner.– this is how scanner and parser get connected.

• If there is no error report, the expression in the input file is correct.

16

Recap

• To write a parser, how many things you need to write?– cup file;– lex file;– a program to use the parser;

• To run a parser, how many things you need to do?– Run javaCup, to generate the parser;– Run JLex, to generate the scanner;– Compile the scanner, the parser, the relevant classes, and the

class using the parser;• relevant classes: CalcSymbol, Symbol

– Run the class that uses the parser.

17

Recap (cont.)

JLex

CalcScanner

javaCup

CalcParser

calc.lex calc.cup

expression

2+(3*5)

tokens

SymbolScanner

CalcScanner CalcParser

lr_parser

implements extends

java_cup.runtime

result

CalcParserUser

18

Evaluate the expression

• The previous specification only indicates the success or failure of a parser. No semantic action is associated with grammar rules.

• To calculate the expression, we must add java code in the grammar to carry out actions at various points.

• Form of the semantic action:expr:e1 PLUS expr:e2 {: RESULT=new Integer(e1.intValue()+ e2.intValue());:}

– Actions (java code) are enclosed within a pair {: :}– Labels e2, e2: the objects that represent the corresponding terminal or non-

terminal;– RESULT: The type of RESULT should be the same as the type of the

corresponding non-terminals. e.g., expr is of type Integer, so RESULT is of type integer.

– In the cup file, you need to specify expr is of Integer type.non terminal Integer expr;

19

Change the calc.cup1. terminal PLUS, MINUS, TIMES, DIVIDE, LPAREN, RPAREN;2. terminal Integer NUMBER;3. non terminal Integer expr;4. precedence left PLUS, MINUS;5. precedence left TIMES, DIVIDE;6. expr::= expr:e1 PLUS expr:e2 {:7. RESULT = new Integer(e1.intValue()+ e2.intValue()); :} 8. | expr:e1 MINUS expr:e2 {: 9. RESULT = new Integer(e1.intValue()- e2.intValue()); :} 10. | expr:e1 TIMES expr:e2 {: 11. RESULT = new Integer(e1.intValue()* e2.intValue()); :} 12. | expr:e1 DIVIDE expr:e2 {: 13. RESULT = new Integer(e1.intValue()/ e2.intValue()); :} 14. | LPAREN expr:e RPAREN {: RESULT = e; :} 15. | NUMBER:e {: RESULT= e; :}

• How do you guarantee NUMBER is of Ineter type?{NUMBER} { return new Symbol(CalcSymbol.NUMBER, new Integer(yytext()));}

20

Change CalcPaserUser

1. import java.io.*;2. class CalcParserUser {3. public static void main(String[] a) throws Exception{ 4. CalcParser parser= new CalcParser(5. new CalcScanner(new FileReader(“calc.input”)));6. Integer result= (Integer)parser.parse().value;7. System.out.println("result is "+ result);8. }9. }

• Why the result of parser().value can be casted into an Integer? Can we cast that into other types?

– This is determined by the type of expr, which is the head of the first production in javaCup specification:

non terminal Integer expr;

21

Calc: second round

• Calc program syntaxprogram statement | statement program

statement assignment SEMI

assignment ID EQUAL expr

expr expr PLUS expr

| expr MULTI expr

| LPAREN expr RPAREN

| NUMBER

| ID

• Example program: • X=1; y=2; z=x+y*2;

• Task: generate and display the parse tree in XML

22

Abstract syntax tree

Program

Statement Statement Statement

Assignment Assignment Assignment

ID Expr ID Expr ID Expr

PLUS Expr Expr

ID MULTI Expr Expr

ID NUMBER

NUMBER NUMBER

X=1; y=2; z=x+y*2;

23

OO Design Rationale

• Write a class for every non-terminal– Program, Statement, Assignment, Expr

• Write an abstract class for non-terminal which has alternatives– Given a rule: statementassignment | ifStatement

– Statement should be an abstract class;

– Assignment should extends Statement;

• Semantic part of the CUP file will construct the object;– assignment ::= ID:e1 EQUAL expr:e2

{: RESULT = new Assignment(e1, e2); :}

• The first rule will return the top level object (the Program object)– the result of parsing is a Program object

• It is similar to XML DOM parser.

24

Calc2.cup1.terminal String ID, LPAREN, RPAREN, EQUAL, SEMI, PLUS, MULTI;2.terminal Integer NUMBER;3.non terminal Expr expr;4.non terminal Statement statement;5.non terminal Program program;6.non terminal Assignment assignment;7.precedence left PLUS;8.precedence left MULTI;9.program ::= statement:e {: RESULT = new Program(e); :}10. | statement:e1 program:e2 {: RESULT=new Program(e1, e2); :};11.statement ::= assignment:e SEMI {: RESULT = e; :} ;12.assignment::= ID:e1 EQUAL expr:e2 13. {: RESULT = new Assignment(e1, e2); :};14.expr ::= expr:e1 PLUS:e expr:e2 {: RESULT=new Expr(e1,e2,e); :}

15. | expr:e1 MULTI:e expr:e2 {: RESULT=new Expr(e1,e2,e); :}16. | LPAREN expr:e RPAREN {: RESULT = e; :} 17. | NUMBER:e {: RESULT= new Expr(e); :}18. | ID:e {: RESULT = new Expr(e); :}19. ;

• Common bugs in assignments: ; {: :}

25

Program class1. import java.util.*;2. public class Program {3. private Vector statements;4. public Program(Statement s) {5. statements = new Vector();6. statements.add(s);7. }8. public Program(Statement s, Program p) {9. statements = p.getStatements();10. statements.add(s);11. }12. public Vector getStatements(){ return

statements; }13. public String toXML() { ... ... }14. }

Program ::= statement:e {: RESULT=new Program(e); :} | statement:e1 program:e2 {: RESULT=new Program(e1, e2); :}

26

Assignment class1.class Assignment extends Statement{2. private String lhs;3. private Expr rhs;4. public Assignment(String l, Expr r){5. lhs=l;6. rhs=r;7. }8. String toXML(){9. String result="<Assignment>";10. result += "<lhs>" + lhs + "</lhs>";11. result += rhs.toXML();12. result += "</Assignment>";13. return result;14. }15.}

assignment::=ID:e1 EQUAL expr:e2 {: RESULT = new Assignment(e1, e2); :}

27

Expr class1. public class Expr {2. private int value;3. private String id;4. private Expr left;5. private Expr right;6. private String op;7. public Expr(Expr l, Expr r, String o){ left=l; right=r; op=o; }8. public Expr(Integer i){ value=i.intValue();}9. public Expr(String i){ id=i;}10. public String toXML() { ... }11.}

expr::= expr:e1 PLUS:e expr:e2 {: RESULT = new Expr(e1, e2, e); :} | expr:e1 MULTI:e expr:e2 {: RESULT = new Expr(e1, e2, e);:}| LPAREN expr:e RPAREN {: RESULT = e; :} | NUMBER:e {: RESULT= new Expr(e); :}| ID:e {: RESULT = new Expr(e); :}

28

Calc2.lex1. import java_cup.runtime.*;2. %%3. %implements java_cup.runtime.Scanner4. %type Symbol5. %function next_token6. %class Calc2Scanner7. %eofval{ return null;8. %eofval}9. IDENTIFIER = [a-zA-Z][a-zA-Z0-9_]*10. NUMBER = [0-9]+11. %%12. "+" { return new Symbol(Calc2Symbol.PLUS, yytext()); }13. "*" { return new Symbol(Calc2Symbol.MULTI, yytext()); }14. "=" { return new Symbol(Calc2Symbol.EQUAL, yytext()); }15. ";" { return new Symbol(Calc2Symbol.SEMI, yytext()); }16. "(" { return new Symbol(Calc2Symbol.LPAREN, yytext()); }17. ")" { return new Symbol(Calc2Symbol.RPAREN, yytext()); }18. {IDENTIFIER} {return new Symbol(Calc2Symbol.ID, yytext()); }19. {NUMBER} { return new Symbol(Calc2Symbol.NUMBER, new

Integer(yytext()));} 20. \n|\r|. { }

29

Calc2Parser User

1.class ProgramProcessor {2.public static void main(String[] args) throws IOException{ 3. File inputFile = new File ("d:/214/calc2.input");4. Calc2Parser parser= new Calc2Parser(5. new Calc2Scanner(new FileInputStream(inputFile)));6. Program pm= (Program)parser.debug_parse().value;7. String xml=pm.toXML();8. System.out.println("result is "+ xml); 9.}10.}

• Debug_parser(): print out debug info, such as the current token being processed, the rule being applied.

– Useful to debug javacup specification. • Parsing result value is of Program type—this is decided by the type of the

program rule:Program ::= statement:e {: RESULT = new Program(e); :}| statement:e1 program:e2 {: RESULT=new Program(e1, e2); :};

30

Another way to define the expression syntax

terminal PLUS, MINUS, TIMES, DIV, LPAREN, RPAREN;terminal NUMLIT;non terminal Expression, Term, Factor;start with Expression;Expression ::= Expression PLUS Term | Expression MINUS Term | Term ;Term ::= Term TIMES Factor | Term DIV Factor | Factor ;Factor ::= NUMLIT | LPAREN Expression RPAREN ;